Conventionally, as obstacle detection devices, a laser radar which uses a laser beam, a microwave radar which uses a microwave, a capacitance type sensor which detects an obstacle from a change in its capacitance, an ultrasonic sensor which uses a ultrasonic wave, etc. are put in practical use. A cheap and reliable ultrasonic sensor, among these devices, is used for a purpose, such as a corner sensor mounted in a vehicle, of detecting an obstacle at a relatively-short distance from the sensor.
Conventionally, many techniques of detecting an obstacle by using an ultrasonic sensor have been proposed. For example, patent reference 1 discloses a technique of transmitting an ultrasonic wave, carrying out a time division process of dividing a transmission pulse time interval by the number of ultrasonic sensors for reception, and switching between output signals by using the two or more ultrasonic sensors for transmission and for reception.